A mammalian pancreas is composed of two subclasses of tissue: the exocrine cells of the acinar tissue and the endocrine cells of the islets of Langerhans. The exocrine cells produce digestive enzymes that are secreted through the pancreatic duct to the intestine. The islet cells produce polypeptide hormones that are involved in carbohydrate metabolism. The islands of endocrine tissue that exist within the adult mammalian pancreas are termed the islets of Langerhans. Adult mammalian islets are composed of five major cell types, the α, β, δ, PP, and ε cells. These cells are distinguished by their production of glucagon, insulin, somatostatin, pancreatic polypeptide, and ghrelin, respectively.
Diabetes mellitus results from the failure of cells to transport endogenous glucose across their membranes either because of an endogenous deficiency of insulin or an insulin receptor defect. Diabetes type 1 is caused by the destruction of β cells, which results in insufficient levels of endogenous insulin. Diabetes type 2, may initiate as a defect in either the insulin receptor itself or in the number of insulin receptors present or in the balance between insulin and glucagon signals, although it is ultimately caused due to a loss of functional β cells. Current treatment of individuals with clinical manifestation of diabetes attempts to emulate the role of the pancreatic β cells in a non-diabetic individual. Individuals with normal β cell function exhibit precise regulation of insulin secretion in response to serum glucose levels. This regulation is due to a feedback mechanism that resides in the β cells that ordinarily prevents surges of blood sugar outside of the normal limits. Unless blood sugar is controlled properly, dangerous or even fatal levels can result. Hence, treatment of a diabetic individual involves monitoring of blood glucose levels and the use of injected bovine, porcine, or cloned human insulin as required. Despite such intervention, there is often a gradual decline in the health of diabetics. Diabetes afflicts millions of people in the United States alone, and there is a clear need to provide additional treatments for this disease.